Many modern flatbed document scanners provide means for scanning transmissive media such as photographic slides or negatives. Transmissive media are sometimes called transparent media, or transparencies. Typically, a transmissive original is placed on or near the scanner platen, and light is directed through the media to be sensed by the imaging system below. A system or device for holding the media and providing the backlight to the media is sometimes called a transparent media adapter.
FIG. 1 shows a typical arrangement in cross section. A 35 millimeter slide 101 is cradled in positioning template 102, which rests on the scanner platen 103. Light source 104 directs light 106 downward through the slide 101 and platen 103, where some of the light is collected by an optical system in an imaging module 105. Often, the scanner is designed to create an in-focus image of an original placed on the platen.
Imaging module 105 may be a contact image sensor (CIS) module. FIG. 2 shows a schematic view of some components of a typical CIS module placed below scanner platen 103. Some parts such as supporting structures have been omitted for clarity. An array of gradient index rod lenses 201 is positioned just below the scanner platen 103. A set of light sensing elements 202 is placed below the array of gradient index rod lenses 201. The light sensing elements 202 are fixed on a circuit board 203, and electrical signals travel from the light sensing elements 203 to other electronics (not shown) through cable 204. Typically, a light source 205 is used to illuminate reflective originals, such as paper documents, that are placed on the scanner platen 103 for scanning. The light source 205 is typically not used when scanning transmissive originals. The CIS module is typically moved by a transport mechanism (not shown) to enable scanning of an area of the scanner platen 103.
FIG. 3 shows the CIS module of FIG. 2 in cross section view. Gradient index rod lenses 201 project an image from an image plane substantially at the surface of the scanner platen 103 onto light sensitive elements 202.
Typically, an imaging module 105 has a limited depth of field. That is, objects placed at the nominal image plane will image sharply onto the scanner's image sensors, but objects displaced significantly from the nominal image plane will appear out of focus in the resulting image. The range of object positions that will result in images of acceptable sharpness is called the scanner's depth of field. A scanner utilizing a CIS module for its imaging apparatus may have a depth of field of 0.5 millimeters or less.
The image, or focal, plane of the scanner may be nominally adjusted to be just above the platen surface in order to fully utilize the scanner's depth of field in the space above the platen where originals or objects can be. However, because the scanner's depth of field is so limited, the focal plane may still be considered to be substantially at the platen surface.
FIG. 4 illustrates some variable dimensions of the transmissive media holding system of FIG. 1. By virtue of the height of the positioning template 102 and the thickness T of the carrier of slide 101, the film portion of slide 101 is held a distance D above the scanner platen 103. Because the scanner is likely designed to focus near the top of the platen 103, slide 101 will be out of focus. Even if slide 101 were placed directly on the platen 103, the film portion would be held a distance away from the scanner's focal plane by virtue of carrier thickness T.
Furthermore, the thickness T of the carrier of slide 101 varies significantly from slide to slide. Some commercially available slide carriers are as thin as 1.2 millimeters, while others are as thick as 3.0 millimeters. Even if the scanner could be configured to focus a distance above platen 103 in an attempt to focus on the film of slide 101, the variability in thickness T imparts enough variability in distance D to carry some slides out of the depth of field of the scanner.
What is needed is a means for scanning, with adequate sharpness, transmissive media on a scanner with limited depth of field.